Voltage comparators are widely used in many electronic devices such as, but not limited to, switches for detecting when a level in a circuit exceeds some particular threshold and in response thereto initiating another action (e.g., turning on another circuit or a load, such as a light or heater, or setting an output level to indicate that a received signal was detected); signal generators for creating a variety of waveforms from triangles; class D amplifiers; pulse code modulators; switching power supplies; and analog to digital converters. Often, the performance of high speed data conversion systems and digital communications receivers is limited by the speed and precision of constituent comparator circuits.
A comparator is simply a circuit which tells which of two input voltages in larger. In general, it generates digital output levels and is required to sense (i.e., "resolve") small differences between the two input signal voltages. To meet these criteria, a large amplification factor is needed. Because of its high gain and stable characteristics, the differential amplifier stage has become the main building block of most comparators. Since linear amplification is not needed, the basic idea is to connect a differential amplifier so that it turns a transistor switch on or off, depending on the relative levels of the input signals. That is, the differential amplifier is operated non linearly, with one or the other of its two transistors cut off at any time.
Of course, this also means that one of the transistors is turned on and drawing current at any time. In some analog to digital converters (ADOC'S), such as flash converters, a large number of comparators may be employed. For example, in an n bit flash converter, the number of comparators may be 2.sup.n-1 or greater. Thus for an 8-bit converter (which is not exceptionally high resolution), 2.sup.7 =128 to 2.sup.8 =256 comparators may be employed. Consequently, it is desirable to reduce the current drawn by each individual comparator not only during dynamic (i.e., switching) conditions, but also during static conditions when that particular state is being maintained.
As high gain circuits, comparators are particularly subject to the influence of noise on input signals. However, noise on the input signals, or either of them, can cause false triggering and erratic switching of the comparator output unless both of the input signals are much larger than the noise level. For that reason, many, indeed most, comparators employ hysteresis. The latter is a form of positive feedback which causes the differential threshold for switching to depend not only on the difference in input voltages, but also on the recent histories of the inputs. Unfortunately, the use of hysteresis is not without adverse effects. Principally, it slows the comparator response, introduces an integrator-like characteristic when enclosed in a feedback loop, and limits the lowest differential voltage which can be sensed.
Since the input signal voltages not only might change rapidly but also are likely to change at different rates (creating an unstable situation for generating a comparison), comparator outputs are often disabled until a safe time after the inputs change to a stable level. Thus, many comparators are provided with, or connected to, strobed latches to capture therein the comparator output at the particular point in time which is of interest, allowing the input signals to change once the latch has been strobed.
Many comparators also suffer from undue coupling between input and output. This causes the input switching threshold to vary with conditions (e.g., changing load impedance) at the output. The digital output of the comparator may, as a consequence, be erroneous.
Accordingly, it is an object of the present invention to provide an improved comparator which draws virtually no current under static conditions.
Another object of the invention is to provide a comparator employing both positive feedback and means for obviating, at least to a large extent, most of the negative effects of hysteresis.
Still another object of the invention is to provide a CMOS comparator which incorporates a strobed latch.
Yet another object of the invention is to provide a comparator with improved input/output decoupling.